I tested these myself for a while and I can't think of any other time I've run in SLI without any heat issues whatsoever despite my having no space between cards. For less than $400 total they also gave about 20% more performance on average than a single overclocked 680. I was really impressed overall.

Those are some great recommendations *cough* give soul a free 660 for SLI *Cough* but I honestly think the 7970 is your best bet in the multi LCD game.

The 7970 should be way more then enough power for 2 or more screens but the 660 is a awesome all round card. I happen to own the MSI 660 TF PE got it as a parting gift when I left the consumer IT industry and love the card but always want more power and when I'm hooked to multi screens it does slow down A LOT! just a heads up